1. Field of the Invention
This invention relates to the manufacture of SiO.sub.2 glass and more specifically to the manufacture of preforms of medium to high purity. In the prior art, this type of material is also dubiously described as "fused quartz" when transparent and as "fused silica" when opaque.
2. Background and Prior Art
Transparent SiO.sub.2 glass is always preferred as bubble-free and homogeneous, in other words as an optical grade of high purity, while the opaque type often is used as a lower cost substitute for the above, comprising a lower purity level. Its opaqueness may simultaneously be a requirement when the transmission of visible light from either direction is t be excluded.
Preforms of SiO.sub.2 glass are sought both as hollow cylinders and as solid cylinders, with precision dimensions for diameter and wall thickness, if hollow, for drawing to form tubing and rod.
For the manufacture of such preforms, both natural and manmade silica materials are utilized. Natural silicas include granular materials derived through physical and chemical benefication from idiomorphic quartz, such as quartz crystals or xenomorphic vein or pegmatite quartzes. For the opaque type, generally sedimentary quartz sands or lower quartz grades of the above quartzes are used. A composite use of these materials is also not uncommon. Among the manmade silicas are those derived as high purity precipitations and depositions from SiO.sub.2 containing solutions and/or vapors.
The tubes and rods produced from the preforms are used in precision shapes for the manufacture of optical components, such as envelopes for high-temperature, high intensity and thus high efficiency lamps. Smaller preforms are used for the manufacture of energy transmitting fibers for optical telecommunications systems.
Another application of the larger preforms is in heat-drawing into large diameter tubes which in turn are utilized as is or converted into high purity containers for use in the manufacture of semiconductor materials, i.e. for holding semiconductor materials in processing steps, such as melting, zone-refining, diffusion or epitaxy. In another variation of their use, the preforms are heat-forged into blocks of different shapes and dimensions from which smaller glass components ar derived by mechanical means, e.g. cutting and grinding.
In the prior art, the manufacture of SiO.sub.2 glass is often equated with mere melting of the quartz. In reality, the manufacture involves multiple sequential phases, all of which can contribute significantly toward the quality and the stability of the glass. I have defined the initial SiO.sub.2 manufacture as having five consecutive basic phases or stages. These phases occur sequentially, either over a prolonged period of time or they may be developed in such short intervals and such rapid succession that they appear as one:
__________________________________________________________________________ Phase 1 Phase 2 Phase 3 Phase 4 Phase 5 Preheating .fwdarw. Melting .fwdarw. Fusing .fwdarw. Equilibrating .fwdarw. Quenching __________________________________________________________________________ .dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw. Conversion of Conversion of Joining of Homogenizing, Cooling, .alpha.-quartz.fwdarw. crystalline glass beads diffusing, stabilizing, cristobalite, into or droplets refining stress relieving outgassing amorphous into larger T &gt;1250.degree. C. T &lt;1050.degree. C. T &gt;1400.degree. C. phase monolith &gt;1723.degree. C. T &gt;1427.degree. C. T &gt;1427.degree. C. __________________________________________________________________________
These phases may then be followed by additional heating phases aiming to achieve specific glass qualities before final shaping by mechanical means, such as cutting and/or grinding.
______________________________________ Phase 6 Phase 7 Phase 8 Re-Heating .fwdarw. Flowing .fwdarw. Quenching ______________________________________ .dwnarw. .dwnarw. .dwnarw. Softening, Forming, reshaping, Stabilizing, homogenizing, blowing cooling diffusing T &gt;2000.degree. C. T &lt;1050.degree. C. T &gt;2000.degree. C. ______________________________________
Allowing for the wide ranges for the temperatures to be used in combination with different atmospheres, gas pressures and mechanical forces, and allowing for from short to prolonged periods of time, the optical quality of the silicon dioxide glass typically undergoes significant changes during the multitude of processing steps as depicted by, but not limited to, the above phases or operative steps.
Upon completion of phase 5, the equilibria within the SiO.sub.2 glass has been defined in phase 4, relative to the temperature, the amounts of dissolved gasses and the bubble content.
In all prior art processes in which the equilibria achieved in phase 1 through phase 5 are permitted to change during or as a consequence of the subsequent treatments of phases 6 through 8, or other past-equilibria treatments, the optical quality of the glass will undergo a change. For example, the prior art processes described in U.S. Pat. Nos. 3,652,245 and 3,674,904 utilize a precharged furnace with an internal resistance heating element for the melting of the surrounding annular layer of granular quartz material, either stationary or with rotation to avoid contact between the heating element and the melt. In both these processes, the melting proceeds radially from the inside to the outside of the granular charge. With the flow of heat, a temperature gradient develops across the thickness of the melt and the melting is thereby non-isothermic. To those skilled in this type of melting, it is known that the temperature on the inner surface of the melt, because of the limitations inherent in the heating element cannot exceed 2000.degree. C., while the outer layer of the melt never exceeds the melting point of cristobalite, i.e. 1723.degree. C.
An inevitable reversal of the above temperature gradient is produced by external heating in the reheating and drawing of the preforms in phases 6 and 7, wherein the temperatures easily exceed 2200.degree. C. As a consequence, bubbles often appear in these zones of the melt formed at lower original equilibrium temperatures. In other words, gases dissolved in the melt at the lower temperatures of the original melt forming temperature gradient become a gas phase when the temperature of the solidified melt, or portions thereof, becomes higher than that at which the gas existed in solution equilibrium in the original melt. Of course, such bubbles are highly detrimental to utilization of the quartz glass in optical applications.
In a similar internal heating and melting process disclosed in GDR Patent No. DD 236,084 A1, the resistance heating element has been replaced with an elongated high powered plasma arc in an attempt to deliver higher temperatures and higher productivity. Unfortunately, while delivering higher temperatures at the inner surface of the melt, it does not do so at the outer surface, with the result that the aforementioned temperature gradient is increased and the process becomes further removed from the isothermic ideal. Identically to the above cited U.S. patents, and despite the higher temperatures of the plasma arc, the highest temperature achieved at the outer layer of the melt is again the 1723.degree. C. melting point of cristobalite.
The specification of GDR D 236,084, suggests that by application of extreme centrifugal forces through rapid rotation of the melt, gas bubbles would be floated to and escape from the inner surface of the melt. However, calculations show that for most of the possible bubble locations within the melt, bubble migration to the inner surface would require unrealistic times. Examinations of such melts confirm concentrated bubble layers approaching the outer surface of the melt, which layers must be removed through wasteful grinding.
The process taught by GDR DD 236,084 use high gaseous pressure within the melting furnace in an attempt to reduce or to eliminate SiO.sub.2 vaporization and to facilitate further superheating of the melt. While the higher temperature favorably decreases the dynamic viscosity of the melt and thus increases the mobility of the bubbles, the higher pressure, intended to reduce or eliminate vaporization, is counterproductive in that it also tends to compress and to reduce the size of the bubbles and thus decrease their mobility which is proportional to the square of their radii. Since the path of an escaping bubble leads to the inner layer of the melt, that path increases as the thickness of the melt grows with time. As length of the bubble escape path grows, so does the time required for its elimination at the inner surface of the melt, finally resulting in the impossibility of escape from the melt.
Despite the powerful plasma arc employed in the GDR patent, its usefulness is preconditioned on the furnace precharge providing the insulation which in turn enables attainment of the higher temperatures. In the event that the arc is accidentally extinguished or needs to be reignited on a finished or near finished melt to continue to melt the balance, despite its power, the furnace may never reach sufficient temperature again.
One of my earlier patents, U.S. Pat. No. 4,188,201, describes another internally heated, horizontally rotating melting furnace for silica materials, comprising an elongated combustion flame burner as well as an elongated resistance heating element. In contradistinction to the above precharged furnace types, the apparatus is designed to provide desirable isothermic melting, by which the melt initially forms as a layer of suitable insulating material and the material to be melted and fused is introduced into the furnace as small quantities at suitable intervals through an elongated feeder coextensive with the burner. While this furnace arrangement provides for isothermal melting and thereby overcomes many of the disadvantages of the precharged furnaces described above, its thermal efficiency and the resultant temperatures are lower (about 2000.degree. C., maximum), because of the heat losses through the water cooled burner. Similarly, the alternate elongated resistance heater also has a lower thermal efficiency. The lower temperatures translate into a low equilibrium temperature. In other words, this prior art apparatus is limited to the production of ingots at temperatures significantly below the reheating temperature employed for drawing or otherwise reshaping the ingot. Further, combustion flames using technical gases such as hydrogen, propane, acetylene, etc., all produce large quantities of combustion products through which heat escapes without being utilized for the melting. They also contain undesirable water vapor which leads to the introduction of large amounts of hydroxyl groups (OH) into the melt. The addition of hydroxyl into the silica glass is equal to an impurity addition, which results in increased tendencies for optical inhomogeneity and low temperature stability of tubes and rods and other components because the hydroxyl groups lower the viscosity of the glass.